When inside walls and outside walls of residences, other buildings and so on are to be newly coated with paint, conventionally, the old paint on the wall may be scraped off the wall, and then stains such as mold growing in the undercoat is washed off with water before the new paint is applied onto the wall surface. In washing the wall with water, a detergent which contains an antibacterial agent is sometimes used. Such a detergent sterilizes the undercoat surface, so when a coating is made on the undercoat, a certain level of asepticization is achieved between the undercoat surface and the film of paint.
However, according to such a method, the coating is susceptible to contamination such as mold attack from exposed surfaces since the coating is formed to cover the undercoat which is asepticized, and there is no treatment made on the exposed surfaces of the paint coating. Once mold grows on the coating, then coated surface can be stained. Further, once mold grows on the coating, the mold may attack not only the coating but eventually the wall material itself, making the problem worse.
In order to prevent or reduce stains and erosions by microorganisms such as mold, there is a method which is already public, of rendering antibacterial capability to inner walls and outer walls of buildings by coating the walls with a paint which contains a powdery optical catalyst. The optical catalyst can be provided by titanium oxide (TiO2) or other semiconductor materials which work as optical catalyst.
Generally, in the semiconductor materials which work as optical catalyst, absorption of a light which has a level of energy equivalent to a band gap between the valence band and the conduction band causes transition of electrons from the valence band to the conduction band. Due to this transition of electrons, the valence band has electron holes. The electrons in the conduction band move to a matter adsorbed on the surface of the optically catalytic semiconductor, and this movement can chemically reduce the adsorbed matter. The electron holes in the valence band get electrons from the matter which is adsorbed on the surface of the optically catalytic semiconductor, and this behavior can oxidize the adsorbed matter.
In titanium oxide (TiO2) which has the optical catalyst capability, the electrons which have moved to the conduction band reduce oxygen in the air, to produce supueroxide anion (.O2−). At the same time, the electron holes in the valence band oxidize water which is adsorbed on the surface of titanium oxide, to produce hydroxy radicals (.OH). Hydroxy radials are highly oxidative. Therefore, if the material which is adsorbed by the optical catalytic titanium oxide is an organic matter for instance, working of the hydroxy radicals may eventually decompose the organic matter into water and carbon dioxide. Among many semiconductor materials which have an optical catalyst capability, titanium oxide in particular works as a superior catalyst in such an oxidation-decomposition reaction as the above, and therefore is used widely in antibacterial agent, deodorants, environmental purification agents, and so on.
A problem, however, is that the titanium oxide optical catalyst itself can only work as a catalyst by absorbing light. For this reason, even if a wall material is coated with a paint which contains titanium oxide that has the optical catalyst capability, antibacterial or antifouling effect based on the optically catalytic decomposition cannot be expected if the wall material is used in a dark place in the building or stored at a dark place where titanium oxide can absorb no or little light. Further, even if the wall material is used at a sunny place, titanium oxide cannot absorb light or enough amount of light during the night time, and so antibacterial effect based on the optical catalyst capability cannot be expected, either.
Further, titanium oxide itself does not have a strong capability to adsorb matters on its surfaces. Therefore, in order to make titanium oxide exhibit its catalytic capability sufficiently, it is necessary to improve contact efficiency between titanium oxide and the target matter which is to be oxidized and decomposed. JP-A 11-343210 and JP-A 2000-1631 for example, disclose techniques for coexistence of titanium oxide and an adsorbent matter in a paint, with an object of improved contact between titanium oxide and the target matter to decompose.
An adsorbent matter known in such an application is calcium hydroxyapatite (Ca HAP). Ca HAP can exchange ions with both cations and anions, and therefore is highly adsorbent. In particular, it is superb in adsorbing organic matters such as protein. For this reason, Ca HAP has been a subject of research in applied technology in many different fields including chromatography adsorbent, chemical sensor, and ion exchanger. However, adding an adsorbent matter such as Ca HAP and titanium oxide separately to a paint and dispersing each of the matters sufficiently is not efficient in manufacture of the paint. Further, making titanium oxide and an adsorbent matter simply coexist in a paint can only make small improvement in contact efficiency between titanium oxide and target matters to decompose.